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Thursday, November 12, 2009

Biodegradable Transistors

Fully biodegradable organic transistors, recently fabricated by researchers at Stanford University, could be used to control temporary medical implants placed in the body during surgery.

Biodegradable chip: After 50 days under conditions that mimic those inside the body, this transistor array is mostly dissolved. Credit: Christopher Bettinger

Biodegradable electronics "open up opportunities for implants in the body," especially if the electronics prove inexpensive, says Robert Langer, institute professor at MIT, who was not involved with the research. Implants might incorporate the organic electronics with biodegradable drug-delivering polymers. Doctors might implant such a device during surgery, then activate it from outside the body with radio frequencies to release antibiotics if needed during recovery. The electronics could also help monitor the healing process from inside the body. After healing is complete, the entire device would dissolve in the body.

Earlier this month, researchers at Tufts University and the University of Illinois at Urbana-Champaign reported building silicon electronics on biodegradable silk substrates. Silicon electronics generally have much better performance than those made from organic semiconductors, but silicon isn't biodegradable. The Stanford group, led by chemical engineering professor Zhenan Bao, is the first to make electronics from fully biodegradable semiconducting materials. Though the devices are stable in water, all that's left of the devices after 70 days are metal electrical contacts just tens of nanometers thick.

So far, the group has proved that it can build organic electronics that work when wet and that break down under conditions that mimic those inside the body. The degradation of these devices is triggered by conditions similar to those found in the body: a salty solution with a slightly basic pH slowly breaks down the transistors. In order to be stable and maintain their performance while they're in use, these devices will need to be encapsulated in another layer whose composition is tuned to expose the device once it has outlived its usefulness. The prototype device, described online in the journal Advanced Materials, is made from biodegradable plastics approved by the U.S. Food and Drug Administration, a biodegradable semiconducting material that resembles the skin pigment melanin, and gold and silver electrical contacts. These metals are also approved for use inside the body.

While the silicon devices might be better suited for longer-term implants such as brain interfaces, where high performance is crucial, fully biodegradable devices might be better suited to applications where it's important for the device to disappear over time, such as tissue engineering or drug delivery, says Bao.

The Stanford researchers next plan to bring down the operating voltage of the devices. Right now it's high enough to split water, which is too high to be safe inside the body. The source of the problem is the insulating layer, or dielectric. In the demonstration devices, the dielectric is an 800-nanometer-thick film of polyvinyl alcohol, which the researchers chose for its biodegradability. But the polyvinyl alcohol layers are thick and tangled, which means the voltage has to be relatively high for electrons to travel through it. The Stanford researchers are currently testing thinner dielectrics, including lipid membranes, that are just tens of atoms thick.

The Stanford group is also testing different materials as substrates for the electronics. The organic electronics are flexible, but the device is built on a brittle plastic. The group will test substrates made of rubbery, stretchy polymers that conform well to biological tissues such as the heart. They're also testing different coatings for the devices. Once exposed to pH levels that mimic those inside the body, the current devices immediately begin to degrade. Bao would like to coat them with materials tuned to dissolve after a desired amount of time.